CN111931370B - COMSOL-based ceramic insulator flash firing method - Google Patents
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Abstract
The method utilizes simulation software COMSOL, takes alumina as a sample, establishes a flash combustion model, obtains the temperature distribution of the surface of the sample, and analyzes the influence of the electric field intensity and the current density on the density and the size of the sample. The method is beneficial to deepening understanding of flash firing, and has important significance for producing high-performance ceramic materials and high-performance ceramic insulators and improving the safety and stability of high-voltage transmission lines in China.
Description
Technical Field
The invention relates to the technical field of ceramic insulator preparation, in particular to a research on a flash firing process for preparing a ceramic insulator by utilizing COMSOL simulation software.
Background
The ceramic insulator has high hardness, high melting point, good chemical stability at high temperature, good electrical insulation, corrosion resistance and the like, and is widely applied to a power grid. However, the ceramic insulator has poor toughness and high brittleness, and is easy to break, and the ceramic insulator is easy to have surface flashover in the operation process according to the field operation. The properties of a ceramic insulator are closely related to its manufacturing process. The traditional sintering process not only needs high sintering temperature and long sintering time, consumes a large amount of energy, but also generates a large amount of greenhouse gas and is very unfavorable for the environment. And the density of the prepared sample is not high enough, which can affect the performance of the ceramic material. In order to solve the above problems, many new technologies have been proposed in recent years. Compared with other novel technologies, the flash sintering has the characteristics of low sintering temperature, high speed, short heat preservation time and the like, so that the flash sintering process is widely concerned.
At present, scholars at home and abroad have deeply studied flash burning process parameters and flash burning mechanisms and obtain a large number of stage achievements. However, students set up flash burning experimental models to analyze the flash burning process. Due to different built experiment platforms, the obtained experiment results do not reach the same. In addition, because the flash burning process is short in time, some parameters of the sample cannot be accurately measured, so that the research on the flash burning mechanism is controversial. Therefore, through establishing the simulation model of the flash combustion, people can conveniently analyze the influence of the experimental parameters, the measurement accuracy is improved, the research of the flash combustion mechanism is hopefully promoted, the actual experimental condition is predicted at first, the cost spent in the actual experiment is reduced, the judgment on the actual experimental result is facilitated, the error or the mistake of the actual experiment is found more quickly, and some dangerous actual operations can be avoided.
Disclosure of Invention
The invention aims to establish a flash combustion model taking alumina as a sample in COMSOL, introduce the steps of establishing a simulation model in detail, obtain the surface temperature of the sample in a stable stage and discuss the influence of electric field intensity and current density on the sample. The research result can be used for researching a flash combustion mechanism, analyzing the influence of factors such as electric field intensity, current density and the like on a flash combustion sample in a flash combustion experiment, opening up a new path for producing high-performance ceramic materials and high-performance ceramic insulators, and having important significance for improving the safety and stability of high-voltage transmission lines in China.
In order to achieve the purpose, the invention adopts the following technical scheme that a ceramic insulator flash firing method based on COMSOL comprises the following specific steps:
1. selection of physical fields
COMSOLMIC is a powerful multi-physical simulation software, and can simulate physical processes in multiple fields. The flash process involves electrical, thermal, etc. physical fields, and thus "joule heat and thermal expansion" physical fields are chosen in COMSOL. The "joule heating and thermal expansion" multi-physical interface involves a combination of thermal, electrical and structural multi-physical effects. The current distribution, the electric field distribution and the potential distribution in the dielectric medium can be calculated in a "current" interface. The "solid mechanics" interface can calculate strain, stress and displacement of the sample. The "solid phase heat transfer" interface can simulate conduction heat transfer, radiation heat transfer and current heat transfer in a model, and the temperature in the interface can cause thermal expansion as the heat load of the "solid mechanics" interface.
2. Establishment of geometric model
The flash-burned sample is a simple three-dimensional geometry, thus creating a cuboid to simulate the sample, which helps us to better understand the flash-burning process, as shown in fig. 1. Wherein the length, width and height of the cuboid are respectively 2.5 μm, 2.5 μm and 1 μm. The two planes along the X-axis direction were fixed, and the material of the sample was set to alumina and the surrounding atmosphere was set to air.
3. Application of boundary conditions
This step is the most critical step in the COMSOL software, and whether the boundary condition setting is reasonable directly determines whether the simulation can be successful. In the flash combustion experiment process, the variables applied externally comprise two variables of a direct current voltage source and a high temperature furnace, so the two conditions also need to be added in the simulation process.
In practical process, the conductivity of alumina will increase with the increase of temperature, so we need to find the conduction current option in the setting window of current conservation under the 'current' interface, and input the relation between the conductivity of alumina and temperature. Thus the current through the sample during the simulation increases with increasing temperature. Two planes along the X-axis are set, one as the input for voltage and the other to ground. In the simulation process, the voltage value can be changed, namely the voltage intensity is changed, and further the influence of the voltage intensity on the sample can be analyzed. The input voltage was set to 0.0375V and 0.075V, and the flash process was studied in both cases.
The initial temperature of the sample was set to 800K at the solid heat transfer node of the model developer window. The purpose of this is to provide an initial temperature of the flash, at which the entire geometric model is maintained. In order to simulate the cooling of the sample with convection air in the flash-burn test, a heat flux boundary condition with a heat transfer coefficient h of 5W/(m2 · K) was set.
4. Partitioning of a grid
The grid of the geometric model directly determines the solving mode of the model, and different grid dividing modes can cause different model solving time, different solving precision and different memory amount required by calculating the simulation model. The finer the grid division is, the higher the solving precision is, and the obtained simulation result is closer to an actual value. Therefore, for different geometric models and different problems, a proper mesh division mode needs to be selected. The simulation accuracy and the simulation time are comprehensively considered, the self-existing tetrahedral mesh is selected for division, and the mesh size setting column is selected to be more detailed. The mesh partition of the geometric model is shown in fig. 2.
The method uses COMSOL to simulate the flash firing process for preparing the ceramic insulator, has important significance for guaranteeing the stability and the reliability of the power transmission line, and has the following specific beneficial effects:
1. although flash firing has been applied and a range of ceramic materials produced, the understanding and study of the flash firing mechanism is limited by the uncertainty of the temperature measurement, and the exploration of the flash firing mechanism is greatly limited by the sharp change and difficulty of measurement of the temperature. The simulation obtains the surface temperature of the flash burning sample in the stable stage, is beneficial to the exploration of the flash burning mechanism, and provides support for the flash burning mechanism proposed by the scholars.
2. The influence of voltage intensity and current density on the density of the sample and the size of the sample is analyzed by utilizing the built flash burning model of the ceramic insulator, the relation between the voltage intensity and the current density is researched, the density of the sample can be really improved by flash burning, and the flash burning model has guiding significance for selecting the voltage intensity, the current density and the temperature in a flash burning experiment.
Drawings
Various other advantages and benefits will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. Also, like reference numerals are used to refer to like parts throughout the drawings. In the drawings:
FIG. 1 is a geometric model diagram of a flash burn simulation;
FIG. 2 is a geometric model mesh partition diagram of a flash simulation;
FIG. 3 is a temperature profile of the surface of a flash-burned sample;
FIG. 4 is a graph of current density over time at different electric field strengths;
FIG. 5 is a graph of surface displacements of flash-burned samples at different voltage levels;
FIG. 6 is a graph showing the surface displacement of the flash sample at a current density of 1.74A/m 2.
Detailed Description
Exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be embodied in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.
1. Flash-burned sample surface temperature analysis:
the surface temperature profile of the flash-burned sample at the stabilization stage when the electric field strength was 300V/cm by changing the DC voltage value is shown in FIG. 3. It can be seen that there is a temperature gradient across the surface of the sample and that the temperature at many locations on the surface of the sample is greater than 800K. Due to the fact that the surface temperature of the sample is increased, the internal void volume is reduced, the particle distance is shortened, the density of the sample is increased, and a stable crystal structure is formed more quickly. This is a good explanation for the short flash time and the rapid densification.
The increase in the temperature of the sample surface can be explained by the joule heating effect. The sample generates joule heat under the action of the current, and the joule heat generates a large amount of heat to raise the ambient temperature. Since the conductivity of alumina is directly proportional to temperature, an increase in temperature increases the conductivity of the sample. When the electric field strength is constant, the electrical conductivity increases, i.e., the resistance decreases, causing an increase in the thermal effect of the electrical energy and an increase in the temperature. This process is a positive feedback and causes the temperature of the sample to rise continuously. The temperature of the sample will increase endlessly if the current density is not limited.
2. Influence of electric field intensity:
when the applied DC voltage was 0.075V, i.e., the electric field intensity was 300V/cm, the change of the current density with time is shown in FIG. 4 (a). According to the simulation 2 result diagram, the flash burning can be divided into three stages: 0-0.25s, 0.25-0.5s, and later 0.5 s. In the first phase, the electric field strength remains constant and the current density and power increase slowly, this phase, called the inoculation phase, is a phase before flash-over occurs. In the second phase, the temperature of the sample is constantly raised by the effect of the joule heating effect, reaching the threshold of flash, so that the sample undergoes flash, rapidly shrinking and accompanied by electroluminescence. Therefore, the second stage is called the flash-burning generation stage, and the most obvious phenomenon in this stage is that the current density rapidly increases. When the current density reaches the critical value, the current density is kept unchanged, and the third stage is entered. The third stage is called the holding stage of flash firing, the current density is not changed any more, and the resistivity of the sample is kept unchanged until the sample is cooled to room temperature.
When the electric field strength was 150V/cm, the current density through the sample was plotted against time as shown in FIG. 4 (b). By comparing fig. 4(a) and fig. 4(b), we can see that the electric field strength has an influence on the incubation time, and the higher the electric field strength, the shorter the incubation time. When the value of the input dc voltage is changed, we find that the displacement maps of the sample surface are the same, as shown in fig. 5. From this result, it can be derived: the electric field intensity can not influence the compactness of the sample.
Thus, the electric field strength does not affect the variation in density of the sample, it only causes the variation in time of the flash inoculation stage.
3. Influence of current density:
the current density is an important factor because it is directly related to the temperature of the sample. The greater the current density, the higher the temperature of the sample. The current density was set to 1.74A/m 2 The displacement of the sample surface is shown in fig. 6. By comparing fig. 5 and fig. 6, we have found that differences in current density lead to differences in sample surface displacement. In other words, the greater the current density, the greater the density of the sample, but the inevitable increase in the size of the sample.
When the chemical components constituting the ceramic insulator are the same, the microstructure of the ceramic is a major factor affecting the performance of the ceramic. In order to improve the insulating property of the ceramic insulator, the performance of the insulator can be well improved by preparing the insulator through a flash firing process. In the flash combustion process, the inoculation time can be adjusted by controlling the voltage intensity, and the density and the size of the sample can be adjusted by controlling the current density.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the appended claims.
Claims (2)
1. A ceramic insulator flash-firing method based on COMSOL is characterized by comprising the following specific steps:
1) selection of physical fields
COMSOLMIC is a powerful multi-physical simulation software, and can simulate physical processes in multiple fields; the flash burning process involves physical fields such as electricity, heat and the like, so that the physical fields of 'joule heat and thermal expansion' are selected in COMSOL; the "joule heat and thermal expansion" multi-physical interface comprises a combination of thermal, electrical and structural multi-physical effects; wherein the current distribution, electric field distribution and electric potential distribution in the dielectric medium can be calculated in a 'current' interface; the "solid mechanics" interface can calculate strain, stress and displacement of the sample; the solid phase heat transfer interface can simulate conduction heat transfer, radiation heat transfer and current heat transfer in a model, and the temperature in the interface is used as the heat load of the solid mechanical interface to cause thermal expansion;
2) establishment of geometric model
The flash-burned sample is a simple three-dimensional geometry, so a cuboid is established to simulate the sample, which helps us to better understand the flash-burning process; wherein the length, the width and the height of the cuboid are respectively 2.5 micrometers, 2.5 micrometers and 1 micrometer; fixing two planes along the X-axis direction, setting the material of the sample to be alumina and setting the surrounding environment to be air;
3) application of boundary conditions
The step is the most critical step in COMSOL software, and whether the boundary condition setting is reasonable or not directly determines whether the simulation can be successful or not; in the flash combustion experiment process, the externally applied variables comprise a direct current voltage source and a high temperature furnace, so that the two conditions also need to be added in the simulation process; in the practical process, the conductivity of the alumina is increased along with the increase of the temperature, so that a conduction current option needs to be found in a setting window of current conservation under a current interface, and the relation between the conductivity of the alumina and the temperature is input; thus the current through the sample during the simulation increases with increasing temperature; setting two planes along the X-axis direction as an input end of voltage, and grounding the other plane; in the simulation process, the voltage value can be changed, namely the voltage intensity is changed, so that the influence of the voltage intensity on the sample can be analyzed; the input voltage was set to 0.0375V and 0.075V, the flash process was studied in both cases; setting the initial temperature of the sample to 800K at the solid heat transfer node of the model developer window; the purpose of this is to provide an initial temperature of the flash, at which the entire geometric model is maintained; in order to simulate the cooling of the sample and the convection air in the flash combustion experiment, a heat flux boundary condition with the heat transfer coefficient h being 5W/(m 2K) is set;
4) partitioning of a grid
The grid of the geometric model directly determines the solving mode of the model, and different grid dividing modes can cause different model solving time, different solving precision and different memory amount required by calculating the simulation model; the finer the grid division is, the higher the solving precision is, and the obtained simulation result is closer to an actual value; therefore, for different geometric models and different problems, a proper mesh division mode needs to be selected; the simulation accuracy and the simulation time are comprehensively considered, the self-existing tetrahedral mesh is selected for division, and the mesh size setting column is selected to be finer.
2. The COMSOL-based ceramic insulator flash firing method according to claim 1, characterized in that:
1) changing the direct current voltage value to obtain a surface temperature distribution diagram of the flash-burned sample at a stable stage; it can be found that the surface of the sample has a temperature gradient, and the temperature of a plurality of parts on the surface of the sample is more than 800K;
2) the electric field intensity does not influence the change of the density of the sample, and only causes the change of the time of a flash inoculation stage;
3) the greater the current density, the greater the density of the sample, but inevitably the larger the sample size.
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